Bottom Contours - Less Is More

janklow · November 17, 2006, 2:58am

Quote:

Lots of talk about speed, drag, and measuring flow under surfboards as a design tool.

How about starting with an aspect of surfboard function which is easily measured. . . . . surfboard speed ?

DrStrange · November 17, 2006, 4:52am

Janklow, that was great. Thanks for posting the link. Now that bird was going “fast”!

ilparadosso · November 17, 2006, 7:02am

Some info on waves to go with the red molecule.

Wave Dynamics [/url]

Looking out at the water, an ocean wave in deep water may appear to be a massive moving object - a wall of water traveling across the sea surface. But in fact the water is not moving along with the wave. The surface of the water - and anything floating atop it, like a boat or buoy - simply bobs up and down, moving in a circular, rise-and-fall pattern. In a wave, it is the disturbance and its associated energy that travel from place to place, not the ocean water. An ocean wave is therefore a flow of energy, travelling from its source to its eventual break-up. This break up may occur out in the middle of the ocean, or near the coast in the surfzone.

In order to understand the motion and beahvior of waves, it helps to consider simple waves: waves that can be described in simple mathematical terms. Sinusoidal or monochromatic waves are examples of simple waves, since their surface profile can be described by a single sine or cosine function. Simple waves like these are readily measured and analyzed, since all of their basic characteristics remain constant.

A simple, monochromatic wave.

Because of their uniformity, simple waves can be readily studied.

Wave Anatomy:

Still-Water Line - The level of the sea surface if it were perfectly calm and flat.
Crest - The highest point on the wave above the still-water line.
Trough - The lowest point on the wave below the still-water line.
Wave Height - The vertical distance between crest and trough.
Wavelength - The horizontal distance between successive crests or troughs.
Wave Period - The time it takes for one complete wave to pass a particular point.
Wave Frequency - The number of waves that pass a particular point in a given time period.
Amplitude - One-half the wave height or the distance from either the crest or the trough to the still-water line.
Depth - the distance from the ocean bottom to the still-water line.
Direction of Propagation - the direction in which a wave is travelling.

The motion and behavior of simple sinusoidal waves can be fully described when the wavelength (L), height (H), period (T), and depth (d) are known. For instance, in deep water - when the depth is greater than one-half the wavelength - wave speed can be determined from the wave size. In shallow water, on the other hand, wave speed depends primarily on water depth.

Similarly, wave height is limited by both depth and wavelength. For a given water depth and wave period, there is a maximum height limit above which a wave becomes unstable and breaks. In deep water this upper limit of wave height - called breaking wave height - is a function of the wavelength. In shallow water, however, it is a function of both depth and wavelength. (Studies suggest the limiting wave steepness to be H/L = 0.141 in deep water and H/d = 0.83 for solitary waves in shallow water.)

Data courtesy of: http://cdip.ucsd.edu/?nav=documents&sub=index&xitem=waves

Breaking Waves

As waves increase in height through the shoaling process, the crest of the wave tends to speed up relative to the rest of the wave. Waves break when the speed of the crest exceeds the speed of the advance of the wave as a whole. Waves can break in three modes: spilling, surging and plunging.

Spilling Waves

The wave crest breaks gradually as the wave travels to the shore. Characterised by the appearance of white water at the crest.

Surging Waves

The wave does not “break” but maintains its basic shape as it moves towards the shore, where it surges up the beach. Very little white water is evident before surging waves reach the shore.

Plunging Waves

The wave crest breaks suddenly and with tremendous force by curling over a near vertical wave face.

Shoaling

The influence of the seabed on wave behaviour. Such effects only become significant in water depths of 60m or less. Manifested as a reduction in wave speed, a shortening in wave length and an increase in wave height.

kcasey · November 17, 2006, 7:55pm

ilParadosso

Even though you were replying to another member’s post, your review was appreciated. Please consider making a contribution to Swaylopedia.

As a next step I would like to introduce some additional material that might not be found in an introductory physical oceanography resource.

Cautionary Note:

Please feel free to pepper the following with ‘IMO’. I always make the mistake of assuming that IMO is a given.

In the hope that it will provide some motivation for getting through (yet another one of my extended) posts, let me begin with my closing statement

“Its all about that decelerating curl, or to put it another way, 'the going is in the decelerating.’”

(On second thought, that might have the opposite effect, oh well.)

When waves shoal (move into shallow water) they slow down; their speed a function of water depth – that is, a breaking wave decelerates. Of course, the reference here is to the waveform (as a whole) and not necessarily to any given water particle. But this deceleration does impact the velocity of the individual water particles as they travel in there ‘loops’ (see your discussion of ‘circular’ paths in your post.)
As a wave shoals there is a conversion of kinetic energy to potential energy –i.e. as the waveform slows, it increases in height. Roughly, this increase in height comes about because (as you have suggested) that the lower regions of the waveform are being slowed ‘faster’ than the upper regions, the upper regions shearing in such a way as to move up and over the slower moving lower regions, to the degree that they can.
As a consequence of 1., the forward speed of the curl region (where the wave is actually breaking or throwing) is moving slower than the shoulder region. (It breaks because that’s where this shearing effect is most dramatic.) Hence the forward velocity of the waveform is slower in the curl than on the shoulder. This of course means that the forward component of the water particle velocity in the curl is less than it is out on the shoulder.
Yet (following point 3.) though the forward component of the velocity of the water particles, relative to that out on the shoulder is less in the curl, the total velocity is usually greater.
The reasons for this curious phenomenon can be attributed to the increase in wave height of a shoaling wave. You might say; as the wave increases in height the water particles have farther to go to ‘get over the hump’ so to speak, hence the upward component of their velocity increases in order to perform this feat. But if attributing volition to water particles disagrees with you, you might just wish to think about the consequences of the shearing effect described in 2., those faster layers ‘lifting’ and sliding up and over the forward slower layers.
I probably should have started with this next point, but I didn’t. So with that out of the way, whereas the net movement of water in a waveform is close to zero - the water does in fact move – or, a passing waveform does amount to flow, though not a net flow. (Shocking, perhaps, but please read on.) It flows one way for the leading part of the wave cycle and more or less the opposite way for the trailing part. Of course, standing back and viewing the whole cycle, the net movement is roughly (as you’ve pointed out) more or less zero. Similarly, stepping back even further, and asking the question “What was that all about?” someone might utter, “Why, that was energy. And it seems to be headed that-a-way.” the individual pointing in the direction of wave’s propagation. That is, there is a flow, it’s just that the net flow is close to zero.
Both of these interpretations, no net motion, or net flow, and energy in transit, have value, especially the later interpretation. However, neither seems to be very helpful in the current context –i.e. surfing and surfboard design. Admittedly, they do provide a sort of basis, or departure point. Which of course leads me to point 6.
With regards to surfing, the leading portion of the waveform is the important ‘bit’. In fact it’s sort of ‘everything’ as far as surfing is concerned. The leading portion or bit referred to is of course that portion of the waveform where the water particles are traveling forward and up. Of course points 5. and 6. should be kept in mind when reading points 1. to 4., but I guess that impossible now. (I really should have opened with 4. and 5. But, what’s done is done.)
Here’s the kicker, and likely to be controversial if anybody actually manages to read this far. If waves didn’t slow down when they shoaled, in particular have that decelerating curl, you couldn’t surf (at least not as it is commonly understood to mean.) That is, and I suspect this doesn’t require explanation, if you catch a swell in the deep ocean, you’re ride will be relatively brief, or to put it another way, your ride will end long before the swell does. You may be able to extend the ride with some interesting design choices, but alas it will end. Yet, not so in the line-up, where you can go as far as the curl will take you, that is your ride will end when the curl does (or prematurely if you so choose, or the choice made for you by some other event.)
Its all about that decelerating curl, or to put it another way, 'the going is in the decelerating.’

kc

PS

My apologies if you knew all this.

TomBloke · November 17, 2006, 9:27pm

So let’s get this straight.

You say that

(1) Non shoaling waves can’t be surfed

(2) Non shoaling waves can be surfed, but it isn’t what we call surfing.

(3) When surfing non shoaling waves the ride will end.

(4) When surfing shoaling waves the ride will not end

(5) When surfing shoaling waves the ride will in fact end, but will go as far as the curl takes you.

(6) This somehow proves that we couldn’t surf unless the curl slowed down.

As someone who regularly surfs waves which don’t curl at all for long distances, I have to disagree. . . . the process of surfing an unbroken wave is essentially the same as surfing a breaking one.

kcasey · November 18, 2006, 5:04am

If by non-shoaling you mean deep-water waves moving at a relatively constant velocity (which is what I meant) then,

(1) Yes – deep-water waves cannot be surfed continuously in the same way a shoaling wave can, the wave will eventually overtake the rider, regardless.

(2) Yes - in that they can be caught and be ever so briefly surfed, but the ride will be short for eventually they (the swell) will overtake the rider (or engulf him) regardless –i.e. nothing he can do, short of paddling or something like that, will keep him out in front of the swell.

(3) Yes again, the deep-water swell will eventually overtake the craft.

(4) Of course shoaling waves end. (You’re a ‘tough crowd’ Roy.) The point I had hoped to make was that for a shoaling wave, the ride ends because the wave ends, not because the wave has overtaken the rider as it does in deep-water.

(5) Yes.

(6) Proves, no, but ‘consistent with’, yes. We are able to surf because of the decelerating curl, actually the deceleration of the complete waveform for that matter. It’s just that this deceleration it is most dramatic in the curl region, in particular it’s where total water particle velocity is greatest.

I actually don’t doubt that you do surf waves that don’t break; I have. But that doesn’t mean they aren’t shoaling. I refer you to ilparadosso’s post; you’d have to be in pretty deep water to be sure a wave isn’t shoaling (assuming it’s not chop, and I’m assuming you’re not referring to chop –i.e. short period waves.)

You actually touched on this a while back in one of your replies to something I wrote. I believe you found it difficult to believe that a wave could push you at the same speed at which the wave was moving. In fact it can’t, for any great length of time, unless it’s shoaling.

In a nutshell, the force on the craft is a function of water particle velocity impacting the bottom of the craft (moving it along as a result) As the craft’s speed increases, the velocity of the impacting water particles (relative to the craft) decrease, and hence the force on the craft (moving it along) decreases. If there was no resistance to motion eventually the craft could be made to travel at the same speed as the wave, but there is resistance to motion, so the craft is always trying to slow down. But when the wave is slowing down too, you’ve got the makings for some surfing. In deep-water the wave doesn’t slow down, it just keeps on going at a constant velocity, and will eventually overtake the craft.

kc

kcasey · November 18, 2006, 6:03am

Also, and I’m not assuming that you’ve accepted my explanations, this all touches on your post about speed and my reply to it – that I don’t believe speed is the issue, but that acceleration and deceleration is.

In particular, and this is likely to get someone upset - drag is a good thing. In fact, in a way, ‘getting the drag right’ makes or breaks a surfboard, so to speak. And by getting it right I don’t mean making it as small as possible. Of course it’s desirable that the design of the board allows the surfer to control drag to the degree to which it is possible, but there is usually a lot of drag present that can’t be controlled (by the surfer’s actions.)

This sort of goes back to that phrase I used in a prior post, ‘holding the wave.’ I wish there was a metric for it - regrettably, it remains a qualitative notion.

If the board has too much drag, it will just loose the wave, or the wave will overtake the craft, or engulf it. If it doesn’t have enough drag, it will forever be bouncing around out in front of the waveform - you’ll never be able to get it to ride on the face. But get the drag right and you can climb, and maintain a position on the face.

A lot of the drag is controlled by the surfer via the presentation of the bottom of the board to the wave, yaw, pitch, roll etc. And, obviously, bottom contours, rockers, fins, etc., will impact how much of an impact his actions will have. But, usually there is a lot of drag present regardless of his actions. The surfers ability to bring drag ‘on line’, and take it ‘off line’ is an interesting way at looking at design.

Anyway, just another bizarre thought for your consideration.

kc

TomBloke · November 18, 2006, 7:42am

Quote:

In a nutshell, the force on the craft is a function of water particle velocity impacting the bottom of the craft (moving it along as a result) As the craft’s speed increases, the velocity of the impacting water particles (relative to the craft) decrease, and hence the force on the craft (moving it along) decreases. If there was no resistance to motion eventually the craft could be made to travel at the same speed as the wave, but there is resistance to motion, so the craft is always trying to slow down. But when the wave is slowing down too, you’ve got the makings for some surfing. In deep-water the wave doesn’t slow down, it just keeps on going at a constant velocity, and will eventually overtake the craft.

kc

Kevin, any surfboard which is riding a wave is already moving beachward faster than the water particles in the top of the wave, so no ‘pushing’ of the surfboard in a horizontal direction is possible.

Thus your musings, which are based on a completely erroneous assumption ( namely, that horizontal beachward water flow pushes the surfboard) , are themselves erroneous.

By the way it isn’t necessary for the wave to be decelerating in order for it to be ridden.

kcasey · November 18, 2006, 11:59am

We disagree.

But (of course) that’s ok.

Fun exchange, thanks.

kc

condition_red · November 19, 2006, 9:25am

Quote:

There’s an absolute motion and relative motion here,
Quote:

Good point.

I had a friend who had the durndest trick. He would push under a pitching wave, do some weird stuff under the water and come flying out the tube.

He entered the situation with zero absolute motion and came out with a bit and a lot of relative motion (tangential to the wave which was, of course, traveling towards the beach).

I’m going to practice bottom turns with the idea that I’m just standing still and the wave is passing under me. I’m sure it will let me attack it more vertically!